How do I know if I am charging enough for repairs?

Work out what one hour on your bench actually earns you, then compare it against what you need that hour to earn. Divide the labour portion of the price by the real time the job takes — door to door, including diagnosis and testing, not the optimistic version. Then reduce it by your re-do rate, because time spent on a job that comes back is time you were not paid for. If the resulting figure is below your target hourly rate, you are undercharging, regardless of how busy the shop is.